The devices mentioned above are optical systems each of which is constructed of a large number of lenses, and these lenses are principally made of glass. In addition, many of the lenses have lens driving mechanisms because the focusing needs to be automatically effected.
As a typical example of such optical systems, the optical disc device will be explained below. The optical disc device has been well known from HITACHI REVIEW Volume 33, Number 4 August 1984, pp. 187-192, "Development of Optical Video Disc and Player".
FIG. 1 shows the general arrangement of the optical system of a prior-art optical disc device. The symbols and operations of various portions in the figure will now be described. Numeral 1 designates a laser diode which serves as a light source. Numeral 2 designates a collimation lens, which turns the light beam of the laser diode 1 into a collimated beam. Shown at numeral 3 is a polarizing beam splitter (hereinbelow, abbreviated to `PBS`), which transmits the output beam of the collimation lens therethrough and which refracts retrogressive light from a .lambda./4 plate indicated by numeral 4 to be stated next. The .lambda./4 plate 4 is used for the phase polarization of light in order to facilitate the discrimination between the input light and reflected light in the PBS 3. Numeral 5 indicates an objective, which is used for condensing input light. Numeral 6 indicates a coupling lens, which receives a beam from the PBS 3 and condenses it. The coupling lens 6 is composed of two semicylindrical lenses which are crossed. Shown at numeral 7 is a photosensor. The photosensor 7 senses the shape of the light spot of input light L6 from the coupling lens 6, thereby to indirectly sense the shape of the light spot of output light L5 from the objective 5. Numeral 8 denotes an actuator, which adjusts the focal position of the output light L5 of the objective 5 in accordance with the output of the photosensor 7. Shown at numeral 81 is a lens driving unit, which adjusts the position of the objective 5 in accordance with a drive control output from the actuator 8. Numeral 9 denotes a disc which can optically record, reproduce and erase information by way of example and which is partly illustrated. The disc 9 is capable of the recording, reproduction, erasing etc. in such a way that the output light L5 from the objective 5 projects a desired light spot on the surface of the disc. Numeral 10 denotes a motor, by which the disc 9 is driven.
FIG. 2 illustrates a prior-art example of the photosensor 7 shown in FIG. 1. In the figure, symbol L6 indicates the output light L6 of the coupling lens 6 in FIG. 1. Symbols P1, P2, P3 and P4 denote photodiodes for converting the quantities of light into electric signals, respectively. Letting V1, V2, V3 and V4 denote the output voltages of the photodiodes P1, P2, P3 and P4 produced when the light spot of the input light L6 is truly circular, respectively, they are set so as to become equal to one another. Numerals 71 and 72 indicate subtracters for tracking on the disc 9, respectively. By detecting the output signal of the difference (V1-V2) between the output voltage V1 of the photodiode P1 and the output voltage V2 of the photodiode P2, whether or not the output light L5 from the objective 5 is projected on a predetermined line is indirectly discriminated. When the output light L5 of the objective lens 5 is not impinging symmetrically with respect to the information recording line of the disc 9, a difference arises between the output voltages V1 and V2. Therefore, the deviation from the recording line on the disc 9 is sensed according to the magnitude and sign of the difference, and a tracking control signal TA1 for controlling the position of the objective 5 is produced until the difference becomes null.
Likewise to the subtracter 71, the subtracter 72 provides a tracking control signal TA2 by receiving the output voltages V3 and V4 of the respective photodiodes P3 and P4 as input signals. Numerals 73 and 74 designate adders, and numeral 75 designates a comparator. They constitute a portion adapted to produce an output signal FA for autofocus control for adjusting the focal depth at which the output light L5 of the objective 5 impinges on the disc 9. More specifically, when the focal depth coincides with the recording line of the disc 9, the input light L6 is in the state of the true circle and enters the photodiodes P1, P2, P3 and P4 equally, and the quantity of the light flux thereof is the largest. The output voltages V1, V2, V3 and V4 are equal to one another, and the magnitude thereof becomes the maximum. On the other hand, in a case where the focal depth has deviated to render the input light 16 elliptical and to cause a difference between values (V1+V3) and (V2+V4) created with the output voltages V1, V2, V3 and V4 of the respective photodiodes P1, P2, P3 and P4, the portion operates so as to eliminate the difference. That is, the value of (V1+V3)-(V2+V4) is detected using the adders 73, 74 and the comparator 75, and the position of the objective 5 is adjusted until the output signal FA becomes zero. FIG. 3 shows a prior-art example of the driving unit 81 for the objective 5 based on the tracking and the autofocus control. Referring to the figure, numerals 811, 812, 813 and 814 designate coils. Shown at numeral 815 is a holder for the objective 5. Numerals 816 and 817 indicate magnets, numerals 818 and 819 springs, and numeral 820 a frame. The objective 5 is fixed by the holder 815, and is installed on the frame 820 of an optical head by the springs 818 and 819. The magnets 816 and 817 are used as ones among elements for determining the position of the objective 5. By making the holder 815 a magnetic member such as iron piece, the position of the objective 5 is determined by the attractive forces of the magnets 816, 817 and the tensile strengths of the springs 818, 819. In accordance with the focus controlling output signal FA from the arrangement of FIG. 2, currents are caused to flow through the coils 811 and 812 so as to move the position of the objective 5 up or down and to establish the desired focal depth. The tracking control signals TA1 and TA2 cause currents to flow through the coils 813 and 814 respectively so as to adjust the lateral position of the objective 5 and to perform the desired tracking. According to the prior-art system described above, the tracking and the focal depth control for accurately projecting the light spot with respect to the recording line of the disc can be carried out.
Further, a prior-art optical disc device is disclosed in Japanese Patent Application Laid-open No. 59-92444 entitled `Pickup for Optical Disc`, laid open on May 28, 1984. This known example replaces an objective with a micro Fresnel lens formed with a concentric grating portion, so as to intensify the diffraction and convergence of light from a light source.
However, the prior-art optical disc devices are very complicated optical systems, and the automatic adjusment of a focus involves the problems of a large number of constituent components and slow response.